High Water and mechanical difficulties knock this ferry service out of service for days at a time. Is there any way to notify Wazers that the ferry is not running and possibly why and for how long ?

Not currently. In the future, from the app, we should be able to call this a road closure. If it is going to be several days, an editor could simply disconnect the ferry segment(s) and Waze won't route over it. Problem is, there's no proactive notification to a driver who doesn't know it's out: Waze will just find another route.